leds: lm3692x: Allow to configure over voltage protection

Overvoltage protection is currently using the default of 29V.  Make it
configurable via DT.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>